The classic symptoms
Active TB rarely arrives suddenly. Its symptoms creep on over weeks, which is exactly why it is so often missed. The classic picture combines a chronic cough with so-called constitutional symptoms — signs that the whole body is fighting a long, draining illness.
- A persistent cough lasting more than 2–3 weeks, often a productive cough bringing up phlegm.
- Drenching night sweats that soak the bedclothes.
- Low-grade fever and unexplained weight loss — the old name 'consumption' came from this wasting.
- Marked fatigue and loss of appetite.
- In more advanced disease, coughing up blood — hemoptysis — as cavities erode small vessels.
Two tests that ask 'have you met TB?'
Before we get to tests that find the bug itself (next guide), there are two that probe the body's memory of TB. They cannot tell latent from active disease on their own — they only show that the immune system has, at some point, recognised the germ. They are workhorses for screening contacts and detecting latent infection.
The tuberculin skin test (TST, or Mantoux test) is the classic. A small amount of TB protein is injected just under the skin of the forearm. The person returns in 48–72 hours, and a nurse measures the firm raised bump (induration) — not the redness. A bump above a size threshold means the immune system reacted. The thresholds vary by risk group, which is why interpretation needs a trained eye.
The interferon-gamma release assay (IGRA) is the modern blood-test version. A blood sample is mixed with TB-specific proteins in the lab; if the person's immune cells have met TB before, they release a chemical messenger called interferon-gamma, which the test measures. It needs only one visit and gives an objective number.
The BCG catch, and what these tests cannot do
The skin test has one famous weakness: the BCG vaccine, given to babies in many countries, can make the TST positive for years even in someone who never caught TB. IGRA blood tests get around this neatly — they use proteins not found in BCG, so a positive IGRA is not confused by past vaccination. This is a major reason IGRA is preferred in BCG-vaccinated populations.